And with its rising star came legions of illegal fishing operations looking to cash in on its success. In 2000, approved fisheries caught more than 16,000 tons of Chilean sea bass. But the National Oceanic and Atmospheric Administration estimates that as many as twice that amount were caught illegally, which has led to troubling overfishing in some areas.

This week Chilean sea bass finds itself back in the news, as the subject of a worrisome new study from the University of Hawaii. Seafood fraud—the substitution of a lesser-valued fish for a more expensive one—is already a known problem, but associate biology professor Peter Marko wondered if the issue unwittingly exposed consumers to higher-than-expected levels of mercury. Using the Chilean sea bass to test his theory, Marko found that fish substitutions “obscure a complex pattern of mercury contamination.”

After DNA-testing Chilean sea bass samples from retail seafood counters across 10 states, Marko found that nearly a quarter of fish given the stamp of approval from the Marine Stewardship Council, a leading authority on seafood sustainability, did not originate from their designated fisheries, but rather from unknown areas where mercury contamination is more likely. MSC-certified Chilean sea bass, in particular, was found to have mercury levels up to three times higher than expected (more than several countries’ import limits).

Yikes! “Although on average MSC-certified fish is a healthier option than uncertified fish, with respect to mercury contamination, our study shows that fishery-stock substitutions can result in a larger proportional increase in mercury,” Marko concluded in a press release.

Timothy Hansen, director of NOAA’s seafood inspection program, assured us that it’s unlikely anyone would get mercury poisoning from eating any sort of Chilean sea bass.

"You just have to eat a lot of these kinds of fish for a long time to get [mercury levels that are] up there," he told us. "I think it’s very unlikely."

At the very least, however, it’s advisable to know what you’re eating, and that’s harder when MSC certification labels aren’t as trustworthy as they seem. MSC did not return our calls as we went to press.

UPDATE:

In response to the above study, the Marine Stewardship Council issued to us the following statement: “When seafood is sold with the MSC ecolabel, businesses and consumers can be assured that the seafood they are buying and consuming is the seafood they purchased. The MSC ecolabel is a trust mark that confirms the seafood bearing the ecolabel can be traced back to a MSC certified fishery. The MSC maintains the most widely recognized global standard for the certification of wild caught seafood as well as a separate Chain of Custody standard to trace seafood. This assures that in every step of the chain, MSC certified seafood is not mixed with or substituted for non-certified seafood.”